A novel approach for cancer treatment is to target the blood vessels (anti-angiogenic therapies) responsible for supplying the tumor with oxygen and nutrients. However, these treatments have not been as effective in clinical studies as originally hoped. It is apparent that tumors are able to evade and escape angiogenesis inhibition, but the mechanism by which this occurs is unknown. We have identified a novel population of tumor cells we hypothesize mediate resistance to anti-angiogenic drugs. These tumor cells express CD31, an endothelial selective marker, and lack the receptors typically targeted by angiogenesis inhibitors. Our lab specializes in studies of tumor endothelial cells (TEC). However, while isolating CD31+ TEC from a model of aggressive melanoma we discovered a unique population of tumor cells which express this endothelial-selective marker. We have determined these cells appear to collaborate with blood vessels in tumors, have endothelial-like characteristics in vitro, yet are positive for melanoma markers. This proposal aims to examine this subpopulation of melanoma and its contribution to the vasculature both in normal tumor settings as well as in tumors which exhibit resistance to anti-angiogenic inhibition. There are two project aims: 1) To further clarify the biological significance of CD31 expression in tumor cells. The goal is to determine the role these cells play in the tumor vasculature as well as elucidate the molecular mechanisms controlling CD31 expression in these cells. 2) To determine how CD31+ tumor cells mediate resistance to anti-VEGF therapies. The goal is to induce anti-angiogenic resistance in models of aggressive melanoma and study the role CD31+ cells play in the induction of this resistance phenotype. This work will undertake a comprehensive study of the significance of these cells in anti-angiogenic resistance using a multi-faceted research strategy. We will examine the characteristics of individual CD31+ cells in vitro to better understand their cellular phenotypes while also conducting in vivo studies of tumor growth and vascular development in conjunction with drug resistance studies. Through the collaboration with Dr. Janiel Shields in the Department of Radiation Oncology at UNC (see letter) we will examine human melanoma cell lines for the expression of CD31. Through another collaboration with Dr. David Ollila in the Division of Surgical Oncology (see letter) we will identfy these CD31+ cells in xenografts of primary human melanoma, and induce anti-angiogenic resistance in these xenografts to examine the role CD31+ melanoma cells play in human models of this phenotype. Overall this study should provide novel therapeutic targets which could be used in conjunction with current anti-angiogenic therapies to improve cancer outcomes.